A large number of short cycles, occurring especially with zoned heating systems, will result in carbonizing the heat transfer surfaces of boilers; and as the heating season progresses will substantially increase the resultant output of pollutants, increase the flue temperature and reduce the boiler efficiency.
The objective of the invention is to attain 15-30% reduction in energy for providing heating and service hot water in buildings which use oil or gas as a fuel and hot water as the heating medium.
A further objective of the invention is to reduce the emittance of pollutants from such heating plants up to 90%.
The above objectives are obtained as follows:
1. By providing means to prevent short cycling of the boiler required for the heating system and to operate it with close to steady-state efficiency rather than with the much lower intermittent efficiency. This also reduces emission of pollutants which is a function of the number of firing cycles.
2. By providing means to transfer fuel energy to the heating medium in minimum space with minimum mass (low inertia), which reduces the energy required for heat-up and minimizes cool down losses.
3. Providing means to heat service domestic hot water from the same boiler which provides heating, without the necessity of maintaining boiler temperature throughout the year or without requiring a separately fired hot water heater. This reduces flue and standby losses and eliminates a separate pilot flame.
4. Providing means for efficient transfer and control of energy to the space to be heated which includes individual zone valves operated by temperature sensors, flow control valves and low water content radiators.
Automatically fired oil or gas boilers operate essentially intermittently with the number of firing cycles depending upon the boiler size, climactic conditions and temperature controls employed. While energy saving zone controls have found wide acceptance in the art, their application also contribute to a considerable increase in firing cycles and thereby offset some of their benefits. A modern zone-controlled residential hot water heating system in a 5000.degree. day area might cycle from 20-30,000 times during a heating season and considerably more if, as is frequently the case, the boiler is oversized for the installation. Such intermittent firing reduces the boiler efficiency from 10 - 40%. With "on" cycles of less than four minutes oil burners have been found to operate at an average efficiency of only 45 - 50% as compared with their steady state efficiency of 75 - 80%.
Aside from reducing the thermal efficiency, short cycling greatly increases the output of pollutants, especially with oil firing, which in turn carbonizes the heat transfer surfaces of boilers decreasing their efficiency as the heating season progresses.
Research conducted by the National Air Pollution Control Administration has demonstrated that the most serious air pollutants from stationary sources result from fossil fuel burning boilers and furnaces; the conclusion reached is that intermittent operation of residential heating systems and many commercial systems create cyclic peaks of carbon particulate, hydrocarbon and carbon monoxide mixtures. These peaks can more than double the output of total pollutants that would result from continuous operation.
While the undesirable effects of short cycling have long been recognized in the art, so far no satisfactory remedy has been found.
Large heating installations frequently employ modulation of the firing rate of the boiler or the modular concept of step firing a number of smaller boilers to meet the fluctuating heating demand. Such systems involve complexities, are only partially effective and are not practical for residential installations.
Another approach to overcome short cycling is to considerably increase the water content of the boiler itself which will enable it to meet short heating demands without firing. Such an arrangement involves however, greatly increased flue losses and the necessity to maintain boiler water temperature throughout the heating season.
The invention resolves the problem by providing additional storage of heating water in a separate well insulated tank outside the boiler itself which is connected in parallel with the heating zones and acts as a load leveler (hereinafter called "heat bank") for the heating system. It banks heat received from the boiler which can be withdrawn by any of the heating zones on demand by the temperature operated zone valves, which in contrast to conventional systems energize only the circulator to withdraw heated water from the heat bank. When 30% of the heat has been withdrawn from the heat bank a wide differential temperature sensor in the thermal bank calls in the boiler which now replenishes the withdrawn heat from the heat bank and simultaneously supplies the heating system itself. In this manner, long operating cycles with steady state efficiency are obtained for the boiler.
By reducing the short cycling of boilers to less then one tenth that of conventional systems, the present invention, in addition to increasing the efficiency of the operation, will thus reduce air pollution to leass than ten percent of the conventional system.